1. Field of the Invention
The present invention relates to loudspeaker systems, and more particularly to loudspeaker systems such as subwoofers for producing frequencies in the lower bass range.
2. History of the Prior Art
Loudspeaker systems such as subwoofer systems designed to produce frequencies in the lower bass range typically employ an enclosure system such as a bass reflex enclosure system which is tuned so that the speaker system can produce frequencies at the lower end of its spectrum without excessive speaker cone excursion and the distortion which typically results therefrom. In a typical bass reflex system, for example, a bass range speaker or woofer is mounted within an enclosure having a port for communicating with the outside of the enclosure. The enclosure is typically tuned to the low frequency range of 30-50 Hz, so that operation of the woofer in this frequency region reduces the amplitude of speaker cone motion and the air velocity in the port increases. The air spring in the enclosure is in resonance with the air mass in the port, and the overall output of the system is the vector sum of port radiation and cone radiation. Because the excursion requirements of the woofer transducer are considerably reduced, substantial output at the port tuning frequency can be realized with low distortion.
Bass reflex speaker systems of this type are not without their disadvantages. At higher frequencies, such as those approximately an octave above port tuning, the contribution from the port is minimal and the output is essentially from the speaker cone. Because most music material has greater program content in the frequency spectrum above 70 Hz, where cone excursion is at a maximum in the bass reflex design, alternative enclosure systems have been investigated.
One such alternative enclosure system is a double tuned bass reflex system in which the enclosure is configured to produce two air masses from a speaker mounted therein. Typically, the speaker is mounted at the front of the enclosure so as to face the interior and thereby produce a first air flow through a first port extending to the outside of the front of the enclosure from the interior thereof. A second port extending into the enclosure from the front thereof communicates with the rear of the speaker to produce a second air flow therethrough.
Another alternative enclosure system is the manifold bass reflex system. The manifold technique closely couples two direct radiators so that the outputs thereof add coherently. The resulting loading and the benefits that derive therefrom are effective up to about 70 Hz. At higher frequencies, the problem of excessive speaker cone excursion still remains. Also, such design requires at least two woofers, and tends to be relatively heavy.
A further alternative enclosure system is the triple chamber bandpass system which utilizes a manifold bass reflex system with the manifold cavity enclosed and ported. The basic configuration is identical to the manifold bass reflex configuration, except that the common cavity is enclosed where the two woofers face each other and a port is added which is tuned to an upper frequency. Two tunings are utilized, one at the low frequency limit of the intended bandpass range (within the outer subenclosures), and the other in a center chamber and tuned to the high frequency limit (below a few hundred Hz). There are two frequency ranges over which enclosure tunings help to reduce distortion by minimizing cone motion. All acoustical output from the system is by way of its port openings, which act effectively as acoustical low-pass filters and further reduce distortion. Dual transducers are employed in a push-push arrangement which reduces mechanical stresses and minimizes third harmonic distortion through cancellation of non-linearities.
It has been found that even with the double tuning of triple chamber bandpass systems, the speaker cone excursions between the lower and upper tunings (in the band between 30 and 50 Hz) can be substantial. A variation of such design has been devised in which one woofer is loaded by a sealed chamber and the second woofer is loaded by a vented chamber, with both woofers facing a common chamber that is ported. Output is by the ported chamber and the common chamber, and the sealed chamber prevents cone over-excursion at frequencies below the lower vent tuning. In addition to requiring at least two woofers, speaker systems of this type also require a relatively large and heavy enclosure, and yet have proven to be no more efficient than simpler designs.
A still further alternative enclosure system is the double tuned quarterwave resonator system in which a single speaker drives two air columns corresponding to quarter wavelengths at frequencies such as 25 Hz and 125 Hz. Each tuned column thus loads the adjacent side of the transducer at a different frequency band, reducing cone excursion for a given acoustic output. The difficulty with this approach is that it becomes impractical for extended bass response. For example, at least one commercially available system of this type is approximately 12 feet in length, with the lower tuned air column comprising approximately 9 feet of that length. Efficiency is relatively poor, frequency response is not very smooth and cone excursions are similar to those present in much simpler designs.
In a still further alternative enclosure system, a compound loaded speaker arrangement is used. There are two variations of this technique, both of which utilize an enclosure with two speakers, one in front of the other. The first speaker faces out of the enclosure, while the second speaker faces into the enclosure. In some designs, the first speaker is located at the end of a short tube, while in other designs, the speakers are mounted directly together, face to face. The Q values of such systems tend to be substantially increased, making it difficult to control transient response unless negative output impedance is used.
Accordingly, it would be desirable to provide an improved speaker system, particularly one designed for use with low frequencies such as in the case of subwoofer applications.
It would furthermore be desirable to provide an improved speaker system in which high output is realized with limited transducer excursion over the intended operating bandwidth. Such speaker system should also desirably provide a relatively smooth response, be relatively efficient, have good sensitivity, and be relatively small in size.